1. Field of the Invention
The invention relates to an extruder for processing powdery bulk materials, comprising a housing; at least one bore that has a bore wall; a feed opening for the powdery material which is disposed on one end of the housing; one screw each that is disposed in the at least one bore wherein said screw has one screw conveyor portion each that is disposed downstream of the feed opening so as to form an inlet zone and at least one portion that is disposed downstream of the screw conveyor portion for processing the bulk material; a metering device that is disposed upstream of the feed opening for feeding in the powdery bulk material and that is designed in a way as to ensure that at least the inlet zone of the extruder is only partially filled during the operation; a vacuum housing portion in the inlet zone wherein said vacuum housing portion has a gas-permeable porous wall portion of sinter material which has a multitude of ducts; and a drive for the at least one screw.
2. Background Art
Such an extruder is known from GB 1 277 150. This disclosure provides a conventional screen or a filter of a sinter material as a vacuum housing portion in order to de-aerate the bulk material to be processed which contains a large amount of air. The porous wall portion may be plane and disposed at a considerable distance from the bores in the housing. Alternatively, said porous wall portion may be disposed in alignment with the bore walls. It has turned out that in the latter case, the wall of the porous wall portion facing the bore was clogged by plasticized plastics adhering thereto, thereby preventing the desired effect of de-aeration. In the former case, de-aeration was stopped almost entirely.
Another extruder is known from DE 195 16 082 A1. This disclosure is based on the fundamental problem that the feeding of powdery bulk material to an extruder is rather difficult. In order to be able to dispense with pre-compressing feeding devices such as plug screw feeders, the inlet zone or solid conveying zone, respectively, is provided with openings whose diameters are so large that they enable gas and solids to be discharged when a low pressure in the range of 0 to 50% is applied. The sole purpose of this low-pressure feeding is the degassing of the bulk material. Another function thereof is to prevent a reverse degassing via the inlet opening which also impairs the feeding of bulk material. Moreover, this allows a pressure build-up to occur as early as in the inlet zone. Another supposed advantage is that an increase of the frictional coefficient occurs on the housing in the vicinity of the openings serving for the discharge of gas and material. A drawback is, however, that some of the powdery solid material is extracted through the openings along with gas. Thus, a sufficient operational safety and efficiency is not guaranteed.
An extruder of the generic type is known from the release of the “The Japan Steel Works, Ltd., Plastic Processing Technology News Letter”, De-Gassing-Cylinder “DGC”—which is equipped with a filter serving as a gas-permeable wall portion through which air but also large amounts of process gas such as water vapour in ABS and wood flour are to be extracted. Here, too, some of the solid material is extracted through the filter openings. Apart from that, there is also the risk that the filter openings might be clogged by the powdery bulk material.
A respective roller press, wherein a screw-type extruder is disposed directly in front of a roller gap, is known from DE 32 20 916 A1 for the compression of powdery or finely crystalline bulk material, wherein the screw-type extruder has a screw housing which has a porous body by means of which a suction opening is formed for the air that is liberated by compressing the powdery bulk material in front of the roller gap. The continuous extraction of air directly in front of the rollers provides the conditions for the desired high compression of the bulk material in the roller gap. Thus, the screw-type extruder serves to pre-compress the powder whilst feeding it under pressure to the roller gap. The screw-type extruder is completely filled with bulk material.